Multilayer targets for calibration and alignment of X-ray based measurement systems

Abstract

Multilayer targets enabling fast and accurate, absolute calibration and alignment of X-ray based measurement systems are described herein. The multilayer calibration targets have very high diffraction efficiency and are manufactured using fast, low cost production techniques. Each target includes a multilayer structure built up with pairs of X-ray transparent and X-ray absorbing materials. The layers of the multilayer target structure is oriented parallel to an incident X-ray beam. Measured diffraction patterns indicate misalignment in position and orientation between the incident X-Ray beam and the multilayer target. In another aspect, a composite multilayer target includes at least two multilayer structures arranged adjacent one another along a direction aligned with the incident X-ray beam, adjacent one another along a direction perpendicular to the incident X-ray beam, or a combination thereof. In some embodiments, the multilayer structures are spatially separated from one another by a gap distance.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]The present application for patent claims priority under 35 U.S.C. § 119 from U.S. provisional patent application Ser. No. 62 / 649,131, filed Mar. 28, 2018, the subject matter of which is incorporated herein by reference in its entirety.TECHNICAL FIELD[0002]The described embodiments relate to X-ray metrology systems and methods, and more particularly to methods and systems for improved measurement accuracy.BACKGROUND INFORMATION[0003]Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishin...

AI Technical Summary

Benefits of technology

[0027]Multilayer targets enabling fast and accurate, absolute calibration and alignment of X-ray based measurement systems are described herein. The multilayer calibration targets have very high diffraction efficiency and are manufactured using fast, low cost production techniques.

[0029]Alignment and calibration performance of multilayer targets as described herein does not depend on the accuracy of the multilayer target geometrical parameters (i.e., height, width, depth). Furthermore, alignment and calibration performance of multilayer targets as described herein does not depend on the accuracy of the method used for target extraction from the multilayer coating initially applied to a substrate. Possible debris and roughness at the periphery of the target does not decrease alignment and calibration accuracy.

[0030]In some embodiments, multilayer targets are employed to align and calibrate a transmission, small angle X-ray scatterometry (T-SAXS) metrology system. Practical T-SAXS measurements in a semiconductor manufacturing environment require measurements over a large range of angles of incidence and azimuth with respect to the surface of a specimen (e.g., semiconductor wafer) with a small beam spot size (e.g., less than 50 micrometers across the effective illumination spot). Accurate positioning of the wafer and characterization of the beam size and shape are required to achieve small measurement box size. In addition, calibrations that accurately locate the probe beam on the desired target area on the surface of a semiconductor wafer over the full range of incidence and azimuth angles are presented herein. This enables precise navigation of the wafer required to measure small box size metrology targets (e.g., metrology targets located in scribe lines having dimensions of 50 micrometer or less).

[0032]In some embodiments, multilayer calibration targets are manufactured using a combination of standard optical multilayer deposition and dicing techniques. This enables fast and low cost production for a wide range of applications.

[0034]In a further aspect, optical markers are fabricated on a surface of a multilayer target that is normal to the incident X-ray beam. The optical markers may be fabricated onto the multilayer target by etching, ion milling, laser scribing, etc. The optical markers enable alignment of the multilayer target with one or more optical microscopes used further for wafer navigation on the X-ray based scatterometry tool.

Problems solved by technology

As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult.

Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty.

For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers.

Optical metrology tools utilizing infrared to visible light can penetrate many layers of translucent materials, but longer wavelengths that provide good depth of penetration do not provide sufficient sensitivity to small anomalies.

As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements.

However, the mirror like structure of 3D FLASH intrinsically causes decreasing light intensity as the illumination propagates deeper into the film stack.

This causes sensitivity loss and correlation issues at depth.

In this scenario, SCD is only able to successfully extract a reduced set of metrology dimensions with high sensitivity and low correlation.

Optical radiation is often unable to penetrate layers constructed of these materials.

As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.

However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).

Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM) are able to achieve atomic resolution, but they can only probe the surface of the specimen.

In addition, AFM and STM microscopes require long scanning times. Scanning electron microscopes (SEM) achieve intermediate resolution levels, but are unable to penetrate structures to sufficient depth.

Thus, high-aspect ratio holes are not characterized well.

In addition, the required charging of the specimen has an adverse effect on imaging performance.

X-ray reflectometers also suffer from penetration issues that limit their effectiveness when measuring high aspect ratio structures.

For example, transmission electron microscopes (TEM) achieve high resolution levels and are able to probe arbitrary depths, but TEM requires destructive sectioning of the specimen.

But, these techniques require sample destruction and lengthy process times. The complexity and time to complete these types of measurements introduces large inaccuracies due to drift of etching and metrology steps.

In addition, these techniques require numerous iterations which introduce registration errors.

Most research groups have employed high-brightness X-ray synchrotron sources which are not suitable for use in a semiconductor fabrication facility due to their immense size, cost, etc.

Current techniques for calibration and alignment of SAXS tools suffer from very long measurement times and their accuracy strongly depends on the accuracy of prepared targets.

Alignment time can be lengthy if the number of required measurement iterations becomes excessive.

In addition, accuracy is limited by the semi-transparency of the knife edge and also strongly depends on manufacturing accuracy of the knife edge.

Characterization of an X-ray beam with traditional knife edges is complicated due to the semi-transparency of the knife material illuminated by X-ray radiation near the edges of the knife edge.

This simple estimate of the uncertainty of a knife edge position during an X-ray beam scan illustrates that when the required alignment accuracy is less than a few micrometers (e.g., less than 10 micrometers), the semi-transparency of the knife edge is limiting.

Unfortunately, errors associated with transferring the measured coordinates from the X-ray camera to the optical microscope are significant and exceed the required accuracy of navigation.

Furthermore, characterization of the X-ray beam by an X-ray camera or knife edges are intrinsically indirect and do not provide quantitative data on photon flux incident on the target as well as photon contamination of neighboring regions.

Diffraction targets manufactured by traditional semiconductor fabrication techniques suffer from low contrast.

In addition, fabrication lead times are usually very long and costly.

A wafer including many targets is very expensive, and any change in target design or target parameter values requires another expensive and long lead time purchase.

Unfortunately, silver behenate targets require very long exposure times and can only be used to perform sample-to-detector distance measurements.

To reduce exposure time, a thicker sample must be used, which increases uncertainty of the distance measurement.

Future metrology applications present challenges for metrology due to increasingly small resolution requirements, multi-parameter correlation, increasingly complex geometric structures including high aspect ratio structures, and increasing use of opaque materials.

Existing methods of X-ray tool alignment and target navigation are limited to an accuracy of approximately 10-20 micrometers.

These methods are not able to position and measure metrology targets of small sizes (−50 micrometers) in an X-ray beam with sufficient accuracy for semiconductor metrology applications.

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